1. Field of the Invention
The present invention relates generally to mobile cellular telephone systems and more particularly to methods and apparatus for overlaying an existing Advanced Mobile Telephone System (AMPS) with a coexisting, transparent, digital network. The digital network may be used for high speed non-interfering data communications, i.e., data communications that do not degrade the underlying system's capacity and capability (qualitatively) to handle voice traffic.
2. Description of the Related Art
Mobile cellular radiotelephone service is presently readily available in many metropolitan areas around the world. The service is typically provided via cellular systems i.e., systems that have a coverage area divided into contiguous smaller coverage "cells" using low power transmitters and receivers. The limited cell area enables the channel frequencies in one cell to be reused in another geographically separated cell according to an established scheme. Thus, a large number of channels can be made available in a metropolitan area and service can appear to be identical to that of a standard telephone.
Cellular systems typically utilize one channel in each cell, called a "control channel", to receive requests for service from subscriber units to call other subscriber units and to instruct subscriber units (also referred to herein as "mobile telephone units") to tune to a frequency pair of a selected "voice channel" wherein a two-way (duplex) conversation may take place. The control channel continuously receives and transmits data and is the channel to which a mobile telephone unit automatically tunes when not in a voice conversational state.
In the prior art cellular telephone systems, telephone calls between two parties continue until their discussions are completed. Since both parties are continuously listening, either can ascertain that the discussions have been terminated and hang up. However, when making data calls on cellular telephone systems, the user is not continuously listening and, as a result, there may be long periods of time when there is no data activity although air time is being utilized. Since the user is billed for the actual air time utilized, the user is being charged at a relatively high rate for such long periods of inactivity.
Many applications, such as electronic mail, would find it advantageous to utilize an AMPS for data transmissions; however, such applications would not want to pay for day long voice calls. Also, a system capacity problem would be likely to result if data terminals (perhaps thousands of terminals coupled to a given network) competed with normal voice users for available system bandwidth.
A lower "bit rate" versus standard air time rates, would be both desirable and practical (for both the user and cellular company), if "scrap" air time could be identified, salvaged and be sold profitably by the cellular company. The existence of "scrap" air time (defined herein as unused and otherwise unusable air time) can easily be demonstrated based on the well known blocking and switching requirements of present day cellular telephone networks.
The U.S. Federal Communications Commission (FCC) requires cellular telephone companies to provide telephone services to subscribers on a 2% blocking basis. This means a subscriber is guaranteed to complete a call on the first try 98% of the time. To achieve this blocking ratio, cellular telephone cell sites have excess capacity in terms of available channels and time. For example, present day cell sites are known that have the capacity to handle 57 channels of voice traffic. Further, it is known that a typical cellular telephone call lasts approximately 2 minutes and that it takes approximately 15 seconds for cell site equipment to transition from one call to another.
The addition of excess capacity due to the blocking factor and transition time represents a considerable percentage of total telephone cell site time availability which cannot be utilized for conventional voice communication. Accordingly, it would, be desirable if methods and apparatus were provided to utilize this time for data transmissions, without interfering with normal voice traffic. Techniques for using this excess and otherwise unusable time, will be referred to hereinafter as "Cellular Data Networking" (CDN) techniques.
Furthermore, it would be desirable to provide a digital network suitable for overlaying an existing AMPS, where the digital network (1) coexists with and is transparent with respect to the AMPS; (2) does not degrade the AMPS's voice capability or capacity to handle voice traffic; (3) utilizes excess and otherwise unusable air time (the aforementioned "scrap" air time), i.e. performs CDN; and (4) utilizes existing AMPS equipment wherever possible to keep the overlay network cost to a minimum.
Many prior art systems are known which attempt to provide means for supporting data traffic in a cellular mobile telephone system, but fall short of providing for the aforestated desirable methods and apparatus.
For example Hess, in U.S. Pat. No. 4,831,373, teaches a method for dynamically allocating data channels on a trunked radio system. More particularly, the method taught by Hess monitors data activity during a predetermined time interval and if the activity is above a predetermined maximum level, additional channels are reserved for data use.
Obviously, voice capacity of an underlying cellular telephone system employing the teachings of Hess, would be degraded as the number of voice channels reserved for data traffic increases. Furthermore, no attempt is made by Hess to utilize excess and otherwise unusable air time to support CDN.
Freeburg et al, in U.S. Pat. No. 4,837,800, teaches a cellular data telephone system that addresses the problems of data calls being charged for long periods of inactivity and freeing up cellular channels assigned to inactive data calls. In particular, Freeburg et al teaches an improved cellular data telephone system and cellular data telephone that are responsive to a lack of data activity (after the expiration of a predetermined time interval) to disconnect a cellular telephone call while maintaining a landline telephone call to a data host. The system taught by Freeburg et al, is also responsive to subsequently occurring data activity by placing another cellular telephone call and reconnecting the landline telephone call thereto.
Freeburg et al does not, however, "yield" to voice traffic, again leading to the possible degradation of voice handling capacity, etc. as the number of data terminals attached to a network increases; nor does Freeburg et al teach, claim or even suggest the utilization of excess and otherwise unusable air time to support CDN.
Felix, in U.S. Pat. No. 4,887,265, is another example of a prior art cellular telephone system that addresses the problem of long periods of data call inactivity on cellular networks and the resulting problems of relatively high data call costs, radio channel spectrum waste, etc. In particular, Felix teaches a packet-switched cellular telephone system having a plurality of packet-switched radio channels each providing packet-switched data services to a plurality of cellular data telephones.
The system taught by Felix also provides for data calls to be "handed off" from one packet-switched radio channel to another on the basis of cellular telephone movement, signal strength, bit-error rate, channel data packet capacity, data packet traffic and/or data packet throughput.
Since cells may be of relatively small size, typically ten miles in radius, the likelihood of a mobile telephone unit travelling out of one cell and into another is high. To maintain uninterrupted communications, the mobile telephone unit is "handed off" between one cell and another. The cell systems track the mobile telephone unit and decide when a "handoff" operation is necessary to maintain high quality communication.
Typically, a mobile telephone unit is commanded by a high speed data message which interrupts audio communications on a voice channel to re-tune the transceiver to another frequency that presently is available in a new cell to which the mobile telephone unit is "handed off". This handoff operation requires a relatively short period of time (for example, 200-700 milliseconds), and the mobile telephone user is usually unaware of the occurrence.
However, when the mobile network is being utilized for data communication, significant data loss can occur during the time interval in which a handoff, power adjustment or similar operation takes place.
Felix recognized that it would be very desirable for owners of portable personalized computers to be able to couple them via a modem, a mobile telephone unit, and a cellular telephone system, to another remote computer. In order to do this and solve the aforementioned problem related to data loss, according to Felix, each cellular telephone in the system has the capability of generating a voice radio channel request to request voice service, and the capability of generating a data radio channel request to request data service. The Felix system includes a plurality of base sites that further include transceiver means having at least one radio signal channel for receiving both the voice radio channel requests and the data radio channel requests from the cellular telephone. However, separate shared data channels (separate from voice channels) are utilized for actual data communications.
Thus, Felix, although addressing (and solving) data call inactivity, handoff problems, etc., fails, like the aforementioned exemplary prior art, to utilize excess and otherwise unusable air time in a system primarily designed to support voice traffic, to support CDN. It should be noted that it would be a desirable characteristic of any methods and apparatus designed to implement a digital cellular overlay network and support CDN, to also provide for maintaining data integrity and avoiding data loss in handoff, power adjustment situations, etc.
Hop, in U.S. Pat. No. 4,912,756, teaches methods and apparatus for error-free digital data transmission during cellular telephone handoff operations, power adjusting operations, multipath fading, etc. In particular, Hop provides a system that includes a program executed on a first computer, that recognizes an imminent "interfering" operation, such as a handoff or the like, and sends a command over a voice channel to thereby blank the voice channel and halt data transmission or reception over the voice channel until the interfering operation is complete.
The completion of the interfering operation is detected by a three-wire bus which in turn allows the computer to recognize the completion of the interfering operation and reinitiate digital data transmission and reception over the voice channel.
Hop does not recognize a demand for voice operations as an interfering condition to interrupt data traffic. Moreover, Hop does not make any determination of unused and unavailable channel capacity for data transmissions; but rather temporarily halts data communications in progress over a given voice channel when an interfering operation is detected.
A further example of the present state of the art is Lusignan, U.S. Pat. No. 4,914,651, which teaches and improved AMPS cellular system where "non-interfering" digital communication channels are added to the existing analog or digital voice communications channels in the system by utilizing the frequency space between channels made possible by AMPS co-channel and adjacent channel frequency coordination procedures.
Lusignan notes that:
"Because of the crowded electromagnetic spectrum used for communications, it is useful to, in effect, squeeze extra communications channels into an already established communications band or system. In the microwave field, extra data channels are accommodated in an FDM-FM voice carrier known as a Data Under Voice service, which is provided by AT&T. Other technologies narrowing channels and allowing closer channel spacing and more capacity have been found to be valuable technological advancements. In addition, there is provided apparatus for multiplexing a number of speech and low speed data channels on a single data multiplex system.
In such techniques as above, technical complexity is required to add the additional capacity without diminishing service on the previous or existing system. This includes, of course, non-interference with the existing channels."
The "space division" technique taught by Lusignan, for supporting data transmissions in an AMPS, suffers from the significant potential for adjacent voice channel interference (although Lusignan is trying to avoid this problem) unless the system is operated at reduced power levels, relatively low operating speeds, utilizing limited bandwidth, etc.
In particular, according to Lusignan:
"Interference is prevented by reduction of power, selection of frequencies, proper modulation and demodulation techniques, and unique allocation to the channels in frequency and location".
Like the other exemplary prior art described hereinabove, Lusignan does not teach, claim or suggest a digital cellular overly network that supports CDN; nor does Lusignan provide a system capable of utilizing, for example, a full 30 Khz of voice bandwidth while supporting full power/high speed data operations in a way that does not interfere with voice traffic.
Notably, Lusignan also assumes a "hex" layout of channels in a cell with no two adjacent channels abutting one another (again to minimize interference problems). It would also be desirable to provide an enhanced cellular data system that realizes the enhanced capacity of Lusignan without having any restrictions on the channel layout in a given cell.
To summarize, in view of the present state of the art as described hereinbefore, it would be desirable to provide methods and apparatus which utilize unused and unusable air time, in an AMPS, for data transmissions without interfering with normal voice traffic supported by the AMPS. In other words, it would be desirable to provide methods and apparatus which perform and support Cellular Data Networking in an AMPS. Such methods and apparatus would provide a mechanism for making lower cost "scrap" air time available for data communications.
Furthermore, it would be desirable to provide a transparent digital network suitable for overlaying an existing AMPS to take advantage of resident AMPS equipment to support CDN, without degrading voice capability or the AMPS's capacity to handle voice traffic, etc.
Still further, it would be a desirable characteristic of any methods and apparatus designed to implement a digital cellular overlay network and support CDN, to also provide for maintaining data integrity and avoid data loss in handoff, power adjustment situations, etc., and to be able to inform data terminals regarding the status of input channels, channel change requirements, etc.
Further yet, it would be desirable to provide a capacity enhanced cellular data system that does not place any restrictions on the channel layout in a given cell and that provides the desired enhanced capacity without placing restrictions on power, bandwidth utilization, speed, etc.